In mechanical design, we use the most is aluminum, in order to prevent rust, anti-scratch, do decoration, etc., often its surface black anodized, or electroless nickel plating treatment.

For example, aluminum alloy 6061, 7075 commonly used surface treatment: oxidation black, color surface treatment, chemical nickel plating and so on.

The terms black anodizing, black plating, and blackening treatment refer to different surface finishing processes for metals, each with its unique characteristics and applications. Here's a brief overview of each process:

Black Anodizing

Black Oxidation: Anodic oxidation is commonly used in aluminum alloy surface treatment, is the use of electrochemical principles, in the aluminum and aluminum alloy surface to generate a layer of transparent aqueous Al2O3 (aluminum oxide) film. Then it is processed by dyeing and sealing. Black is a very common anodic dyeing, very mature process. The anodized film has excellent abrasion and corrosion resistance.

  • Process: Black anodizing is an electrochemical process used primarily on aluminum parts. It involves immersing the aluminum in an acid electrolyte bath and passing a current through the medium. This results in the aluminum surface oxidizing and forming a layer of aluminum oxide.
  • Characteristics: The anodized layer is very hard, durable, and resistant to corrosion and wear. The thickness of the anodized layer can be controlled, and it can also provide better adhesion for paints and glues. The "black" in black anodizing comes from dyeing the porous anodized layer or using electrolytic coloring to achieve a deep black finish.
  • Applications: Widely used in aerospace, automotive, consumer electronics for parts that require enhanced durability and corrosion resistance.
Finishing-of-Hard-Anodized-Aluminum-Parts

Classification of Anodizing

Anodizing is a process that enhances the corrosion resistance, durability, and appearance of metal surfaces, especially aluminum. It involves using an electrolytic passivation process to increase the thickness of the natural oxide layer on the surface of metal parts. The classification of anodizing can be broken down based on the standards and specifications it meets, as well as the desired end-use of the product. Here are the primary classifications:

Type I - Chromic Acid Anodizing

  • Process: Uses chromic acid to create a thin oxide layer.
  • Characteristics: Produces a thinner anodized layer (0.5 to 1 µm) compared to other types, offering the least amount of surface dimensional increase. It is less abrasion-resistant but provides excellent corrosion protection and is used when retaining fatigue strength is critical.
  • Applications: Ideal for aerospace and applications requiring minimal dimensional changes.

Type II - Sulfuric Acid Anodizing

  • Process: Utilizes sulfuric acid to produce a thicker oxide layer than chromic acid anodizing.
  • Characteristics: The anodized layer typically ranges from 1.8 to 25 µm. It provides good wear and corrosion resistance and can be dyed in various colors, making it the most commonly used anodizing type.
  • Applications: Widely used for architectural, automotive, consumer goods, and aerospace components where aesthetic appeal and durability are required.

Type III - Hard Anodizing (Hardcoat)

  • Process: A variation of sulfuric acid anodizing, conducted at lower temperatures and with higher current density, resulting in a much thicker oxide layer.
  • Characteristics: The anodized layer can be from 25 µm up to 150 µm. This type offers superior wear and corrosion resistance, along with increased durability. The surface can still be dyed, though the colors might be less vibrant compared to Type II.
  • Applications: Suitable for components subjected to extreme wear or harsh environmental conditions, such as military, industrial, and some aerospace applications.

Hard anodizing can be achieved by electrolysis using a mixture of sulfuric and oxalic acid at higher concentrations, and lower temperatures (0-10°C). The oxide layer is 25um-250um thick, the coating is grey to black and non-porous, and usually has a hardness of 500-900 HV, which enhances the wear resistance of the aluminum alloy surface. In addition, not all aluminum alloys can be anodized.

For example, the 5xxx, 6xxx and 7xxx series of aluminum alloys respond well to hard anodizing, while the 2xxx aluminum alloys and other alloys, including cast aluminum alloys with high copper and silicon content, respond poorly to anodizing, and for these alloys, the anodized layer tends to be porous and of low hardness. Hard anodizing, which is done at lower temperatures, is more expensive and harder, but is generally indistinguishable to the naked eye and can be distinguished by doing tests or processing.

Black Plating

Black plating: Plating has various treatment methods such as zinc plating, nickel plating, chromium plating, gold plating, and silver plating. It is a surface processing method in which the cation of the metal to be plated is deposited on the surface of the base metal by electrolysis in a salt solution containing the metal to be plated, with the base metal to be plated as the cathode, and the plated layer is formed by depositing the cation of the metal to be plated on the surface of the base metal by electrolysis. The purpose is to play a decorative, anti-rust, wear-resistant role.

  • Process: Black plating involves depositing a layer of black material onto the surface of a metal through various techniques, including electroplating (where a metal is deposited via an electric current) or electroless plating (chemical deposition without electricity). Common materials used for black plating include black nickel, black chrome, and black zinc.
  • Characteristics: The resulting finish can provide a degree of corrosion resistance, improved aesthetic appeal, and some wear resistance, depending on the plating material used. The thickness of the plating layer is typically thin.
  • Applications: Black plating is often used for decorative purposes, as well as in some automotive, hardware, and electronic applications where a black finish is desired for aesthetic or functional reasons.
Black electroplating

What is the difference between anodizing and plating?

Anodizing and plating are metal finishing processes that enhance surface properties, but they operate through distinct mechanisms. Anodizing involves an electrochemical process that converts the metal surface into a durable, corrosion-resistant oxide layer. Primarily used for aluminum, this method thickens the natural oxide layer, offering enhanced protection and the option for colored finishes through dyeing.

In contrast, plating deposits a thin layer of another metal or alloy onto the surface of a metal through electrochemical, chemical, or physical methods. Plating can be applied to various metals, enhancing attributes like corrosion resistance, electrical conductivity, and aesthetic appeal. While anodizing integrates the protective layer into the base metal for increased durability and wear resistance, plating adds a separate layer that can sometimes peel or flake. The choice between anodizing and plating hinges on application-specific requirements, including the desired finish, environmental exposure, and the base metal involved.

Black Oxide

Blackening: Blackening treatment is to put the metal parts in a very concentrated alkali and oxidizer solution heating oxidation, so that the surface of the metal parts to generate a layer of magnetic iron tetraoxide film, commonly used in low carbon steel, low carbon alloy tool steel. Due to the influence of materials and other factors, the film color of the blackening layer has blue-black, black, red-brown, brown, etc., and its thickness is 0.6 ~ O.8µm, the film layer of the blackening treatment has almost no rust-proofing ability, and on the basis of additional oiling, the salt spray test can only pass 3-5 hours.

  • Process: Blackening, or black oxide coating, is a conversion coating for ferrous metals, stainless steel, copper, and copper-based alloys, zinc, powdered metals, and silver solder. It is achieved through a chemical reaction between the iron on the metal surface and the oxidizing salts present in the black oxide solution. This process produces a black iron oxide (Fe3O4) layer.
  • Characteristics: The black oxide coating provides a matte black appearance and offers mild corrosion resistance, reduces light reflection, and minimizes dimensional changes to the part. However, it's less durable and protective than anodizing or plating and is often supplemented with a post-treatment oil, wax, or lacquer to improve its corrosion resistance.
  • Applications: Commonly used for automotive parts, gears, fasteners, and tools to reduce glare and improve lubrication adherence, without significantly altering the dimensions of the part.

Blackening involves a chemical transformation that creates a coating on metal surfaces, distinct from methods like nickel or zinc plating where the coating is externally applied. This process occurs when iron on the surface of ferrous metals reacts with oxidizing salts in the black oxide solution, incorporating elements like penetrants, catalysts, activators, and specific additives into the reaction. This interaction results in the formation of a black oxide layer directly on the metal's surface.

Used predominantly on ferrous alloys such as carbon, alloy, tool, cast steels, and stainless steels, blackening can also be applied to certain non-ferrous metals like copper and brass under appropriate conditions to achieve a similar black oxide coating. Unlike anodizing, which is suited for aluminum, magnesium, and titanium alloys and relies on an electrochemical process requiring electricity, blackening is achieved through chemical reactions without external power.

Blackening is categorized into thermal (>140°C), medium temperature (107-118°C), and room temperature (13-29°C) processes, each offering unique advantages. Thermal blackening generates a durable Fe3O4 layer suitable for a broad range of ferrous alloys. Medium temperature blackening is environmentally friendly, avoiding corrosive fumes, while room temperature blackening, favored for its safety and ease, yields deep black finishes and corrosion resistance comparable to thermal methods. It's especially effective for powdered metals and cast iron.

Following blackening, metals are often immersed in oil or wax to enhance corrosion resistance, providing a cost-effective, matte black finish with numerous benefits:

  1. Decorative and stable black color.
  2. Minimal dimensional change, with a coating thickness of about 0.75µm.
  3. Strong adhesion to the substrate, preventing peeling.
  4. Improved anti-wear properties and lubrication.
  5. Reduced light reflection for parts, decreasing eye strain.
  6. Enhanced, though limited, corrosion resistance.
  7. Cost-efficiency compared to other coatings.

This treatment is widely applied to a variety of products including fasteners, tools, machine parts, medical instruments, automotive components, and sports equipment, offering both aesthetic and functional advantages.

black-oxide-finish

Black Anodizing vs. Black Plating vs. Black Oxide:

Black anodizing, black electroplating, and blackening treatment each offer unique benefits for metal surface treatment.

  • Black anodizing excels in enhancing aluminum alloy parts' corrosion resistance and durability, widely used in aerospace and automotive industries.
  • Black electroplating, less common, provides decorative and protective qualities with a distinctive yellowish-brown appearance.
  • Blackening treatment, focusing on stress relief and material property improvement, is mainly applied to low-carbon steel and low-alloy tool steel, offering minimal corrosion resistance.

Each method serves specific applications, making the choice crucial for desired performance and appearance outcomes.

Property

Black Anodizing

Black Electroplating

Blackening Treatment

Materials

Aluminum alloys

Various metals

Low-carbon steel, low-alloy tool steel

Applications

Aerospace, automotive

Decorative, protective

Engineering, stress relief

Characteristics

Corrosion-resistant, durable

Decorative yellowish-brown, protective

Minimal corrosion resistance, stress relief

Black-Anodized-Aluminum
Black electroplating
black Oxide
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